Temperature compensation in many types of resonators is often accomplished by utilizing metals with different thermal coefficients of expansion. The desired effect is that as one structural part expands or contracts with temperature the other parts do the same to a larger or lesser degree such that the resonant frequency of the resonator remains within certain specified limits.
While the prior art shows many examples of temperature compensated cavity resonators, this cannot be said of helical resonators. A helical resonator consists of a helical coil (helix) wound on a dielectric form spaced from and surrounded by an electrical shield. It is simply a parallel tuned circuit in which the lumped inductor, the helix, resonates with the distributed capacitance from the helix to the shield. By choosing the helix and shield materials to have different thermal coefficients of expansion, the increase in inductance of the helix with temperature could be compensated for by the decrease in distributed capacitance from the helix to the shield, as the latter also expands. The ratio of the two thermal coefficients of expansion necessary would depend primarily on the geometry of the resonator (helix and shield).
The present invention provides a temperature compensated helical resonator structure, that also exhibits drop or shock resistance. Shock resistance is achieved by providing in the otherwise vacant space between helix and shield a rigid dielectric filling. However, the simple introduction of a filling dielectric would interfere with the temperature compensation that has been achieved, due to the dielectric material's own behaviour with temperature. As will be shown in conjunction with the detailed description later on, it is at best impractical to try to find a combination of two metals plus a third material that would satisfy the conditions for thermal stability. It was found, however, that by providing the dielectric filling in such amount to yield a desired average (i.e. effective) permittivity of the space between helix and shield, temperature compensation is enhanced in addition to the achievement of additional support for the helix against shock. The filling dielectric in most cases must have a negative thermal coefficient of permittivity, the value of which determines the amount of filling (i.e. fill factor).
As a result of this improvement it is now possible to have temperature compensated, shock resistant, helical resonators made from a single metal, for example copper which is a good conductor in addition to being inexpensive and practical to work and machine. It is often more practical, however, to use a material such as brass (or aluminum) for the housing and conventional copper wire for the helix, since brass is even easier to work and machine than is copper.
Thus, according to the present invention there is provided a helical resonator structure comprising a housing having inside walls and an end portion, and being of a conductive material having a first positive thermal coefficient of expansion; a form of dielectric material inside said housing spaced from the inside walls and supported by said end portion, said form supporting a thereon wound conductive helix of a material having a second positive thermal coefficient of expansion; and rigid dielectric filling disposed in selected locations contiguous said helix and the inside walls in the space therebetween, said dielectric filling being of a predetermined amount to produce an average permittivity of the total of said space, that is larger than that of air but less than that of dielectric filling material, and said dielectric filling material having a negative thermal coefficient of permittivity.